The invention relates generally to the field of digital image processing and, more particularly, to a method for correcting distortion in an image.
Typically, cameras have a flat image surface. That is, the film or image sensor is typically flat. Aberrations in the optical system can produce field curvature. Field curvature describes the condition where the focal surface of the optical system is actually a curved surface rather than planar.
A camera having a curved image surface was described by H. Casler in U.S. Pat. No. 1,082,678. Casler disclosed a camera using flexible film and mechanically forcing the film negative to conform more closely to the focal surface than is typically accomplished with a flat film image surface, thereby producing improved image quality.
The curved image surface is advantageous because it compensates for the optical aberrations of field curvature and distortion. Distortion refers to the variation of the image magnification with the position in the image. U.S. Pat. Nos. 2,479,553; 2,544,864; 3,006,248; 4,932,764 and 5,000,552 all describe cameras having optical systems with curved image surfaces. In general the curved image surface is cylindrical, which is an approximation to the ideal spherical image surface. The manufacture of a spherical image surface is not practical because film is manufactured in flat sheets and with bending can conform to a cylindrical shape, but is incapable of conforming to a spherical shape without tearing or wrinkling the film. Alternatively, it is extremely difficult to manufacture film that is not flat.
Single-use cameras (sometimes called lens-fitted film) have been popular for the reason that they provide an inexpensive means for individuals to practice photography. These cameras often employ a curved (cylindrical) image surface with a fixed position lens to partially correct for several optical aberrations, including field curvature and distortion. Having the cylindrical image surface for the film negative partially compensates for the field curvature and distortion of the optical system. However, because a cylindrical (rather than spherical) focal surface is used, the effects of distortion are different in each image dimension. In the dimension parallel to the axis of the cylindrical focal surface, the distortion is worse than in the other dimension. Thus, the distortion is not radially symmetric. The remaining distortion is still of large enough magnitude to adversely affect the quality of the image. Koike, in U.S. Pat. No. 6,349,171, describes a specific design for the lens of such a camera to reduce the effects of optical aberrations other than distortion. The distortion of a digital image scanned from an image captured from such a system then is digitally processed to remove distortion. While Koike describes a method of reducing the processing time to produce a pleasing digital image, an efficient means for performing the distortion correction is not described.
As described by Koike, distortion in an image may be corrected digitally. However, it is generally assumed that the distortion in an optical system is radially symmetric, and the distortion magnitude can be represented by a single parameter. (See, for example, Toyoda et al. in U.S. Pat. No. 5,461,440.) It is difficult to simultaneously correct for the distortion in both dimensions of an image captured with a cylindrically curved image surface with a symmetric model of image distortion.
Alternatively, both Toyoda et al. and Florent et al. (in U.S. Pat. No. 5,675,380) also describe that the distortion of any optical system can be determined and corrected by imaging a target and finding the difference between the image of the target and an ideal image of the target (derived from a model of perspective projection). An image of correction vectors can be generated by comparing the image of the target and an ideal image of the target. The correction vectors are used to geometrically transform (warp) any image from the optical system to be free of distortion. While this method can indeed correct image distortion, even non-symmetric distortion, it has several drawbacks. First, the correction vectors represent a significant amount of data, and storage of the correction vectors requires significant system resources. Secondly, this method does not allow for the correction of distortion of an image if the optical system from which the image was produced has not first imaged the target. Furthermore, the huge number of correction vectors is too many for an operator to manually optimize.
Therefore a need exists for a method of correcting a non-symmetric distortion in images which has low memory requirements and does not require target images to first be imaged with the optical system.
The present invention is directed to overcoming one or more of the problems set forth above. Briefly summarized, according to one aspect of the invention, a method for correcting for non-symmetric distortion in an image generated from an imaging system producing a distortion predominantly in one image dimension includes the steps of: providing a distortion model capable of correcting the non-symmetric distortion in the image that is predominantly in the one image dimension; providing one or more correction parameters, wherein at least one correction parameter relates to the distortion predominantly in the one image dimension; and generating an image corrected for the non-symmetric distortion by using the distortion model and the one or more correction parameters.
In a further aspect of the invention, a system for correcting non-symmetric distortion in an image generated from an imaging system producing a distortion predominantly in one image dimension comprises: a distortion corrector utilizing a distortion model capable of correcting the non-symmetric distortion in the image that is predominantly in the one image dimension; a source of one or more correction parameters, wherein at least one correction parameter relates to the distortion predominantly in the one image dimension; and means for inputting the correction parameters into the distortion corrector, thereby generating an image corrected for distortion by using the distortion model and the correction parameters.
One of the advantages of the invention is that it provides a simple and efficient way to improve images produced by inexpensive cameras, such as single use cameras, that employ a cylindrical imaging surface to partially compensate for distortion caused by inexpensive fixed focal length lenses. Such cameras produce images that compensate to some extent for distortion, such as barrel distortion, in one image dimension but leave a non-symmetric distortion, such as pincushion distortion, in the other dimension. The present invention provides an advantageous way to correct for the remaining non-symmetric distortion. Moreover, correction can be provided in both dimensions, as necessary or desired in specific cases to further improve the image.
These and other aspects, objects, features and advantages of the present invention will be more clearly understood and appreciated from a review of the following description of the preferred embodiments and appended claims, and by reference to the accompanying drawings.